1. Field of the Invention
The present invention relates to a liquid crystal display device having the function of displaying intended display data as image data at selected ones of a plurality of pixels constituting a liquid crystal display panel, a drive circuit for the liquid crystal display device for supplying a power source voltage required for displaying the display data, and a method for driving the liquid crystal display device.
More particularly, the present invention relates to a liquid crystal display device comprising at least one gate driver for sequentially scanning a plurality of pixels through a plurality of first bus lines (generally called scan bus lines), and at least one data driver for supplying a drive voltage for displaying the display data to selected ones of the pixels on the first bus lines through a plurality of second bus lines (generally called data bus lines) crossing the first bus lines, a drive circuit for the liquid crystal display device for supplying the data driver with a power source voltage required for displaying the intended display data by activating the data driver, and a method for driving the liquid crystal display device for supplying the power source voltage to the data driver.
Generally, a portable personal computer such as a notebook-sized personal computer is used with a liquid crystal display device (usually abbreviated to LCD) as a thin and lightweight display device. Especially, in recent years, the notebook-sized personal computer or the like has rapidly prevailed, and opportunities for using various products associated with the notebook-sized personal computer or the like with a liquid crystal display device built therein have increased not only in workshops or offices, but also in various places other than these workshops or offices. Therefore, there is a tendency for high demand for various products to which the liquid crystal display device is applied, using a charge-discharge battery having a longer operation time, and smaller and lighter products to which the liquid crystal display device is applied, having a smaller charge-discharge battery. The current trend is to meet this demand and realize a liquid crystal display device which consumes less power by saving unnecessary power consumption as far as possible.
2. Description of the Related Art
In order to facilitate an understanding of the problems of the conventional liquid crystal display device used with the notebook-sized personal computer and the data driver of the liquid crystal display device, an explanation will be given of an example configuration of the conventional liquid crystal display device used with the notebook-sized personal computer and the operation of the data driver of the liquid crystal display device with reference to FIGS. 1 and 2 as described later in "BRIEF DESCRIPTION OF THE DRAWINGS".
In FIG. 1, a block diagram showing a configuration of a conventional liquid crystal display device is shown. In FIG. 2, a voltage waveform diagram showing the manner in which upper positioned data drivers and lower positioned data drivers, of the conventional type, are controlled is shown. The description that follows refers to a typical case in which the data drivers are divided into those including a first upper positioned data driver 600-1 to an N-th upper positioned data driver 600-N (N: an arbitrary positive integer, hereinafter referred to simply as the upper positioned data drivers) connected to every other second bus line (for example, odd data bus lines) 60a, and those including a first lower positioned data driver 610-1 to an N-th lower positioned data driver 610-N (hereinafter referred to simply as the lower positioned data drivers) connected to the remaining every other second bus line (even data bus lines, for example).
The liquid crystal display device shown in FIG. 1 comprises a plurality of first bus lines 50, a plurality of second bus lines 60a, 60b crossing the first bus lines 50, and a plurality of pixels formed at the intersections of the first and second bus lines on a liquid crystal panel 4 which is configured of a m.times.n matrix (m, n: arbitrary positive integer). In other words, a plurality of liquid crystal cells constituting a plurality of pixels representing the minimum units of display data are arranged at the intersections of a plurality of first electrodes (row electrodes) X1 to Xm making up the first bus lines 50 (scan bus lines) and a plurality of second electrodes (column electrodes) Y1 to Yn constituting the second bus lines 60a, 60b (data bus lines).
Further, these pixels are connected to a plurality of transistor switching elements (not shown) made of TFTs (thin film transistors), respectively. These transistor switching elements in turn are connected to a plurality of the second electrodes Y1 to Yn, respectively. Furthermore, control gates for controlling the on-off operation of the transistor switching elements are connected to the first electrodes X1 to Xm, respectively. If it is assumed that the transistor switching elements are turned on and off by applying a predetermined voltage from a plurality of the first electrodes X1 to Xm and a plurality of the second electrodes Y1 to Yn, the same voltage is applied to the corresponding pixels through the transistor switching elements thus turned on, and an electric field is exerted on the liquid crystal in the pixels, and the orientation of the liquid crystal is changed. Thus the intended display data can be displayed as image data.
In FIG. 1, a horizontal arrangement of pixels, i.e. an arrangement of pixels along each scan bus line is called a line. The display data on the liquid crystal panel 4 is written for each scan bus line. This operation is repeated at the rate of 60 times per second, thereby presenting a flickerless image to the eyes of the user. Typically, a liquid crystal display device which has about 640 (n=640) pixels along the horizontal direction (along the scan bus lines) and about 480 (m=480) pixels along the vertical direction (along the data bus lines) of the liquid crystal panel 4, finds wide application. Further, the color display requires additional pixels for R (red), G (green) and B (blue).
The liquid crystal display device further comprises a gate driver 5 for supplying each of the first electrodes X1 to Xm with a gate driver control signal Sxco used for the on-off operation control of the transistor switching elements after being converted to an appropriate voltage level. The gate driver 5 causes all the pixels on the liquid crystal panel 4 to be sequentially scanned through the respective transistor switching elements. There are usually provided a plurality of gate drivers 5 including a plurality of ICs (integrated circuits) connected to the first electrodes X1 to Xm, respectively. In FIG. 1, however, only one gate driver is shown for simplifying the explanation about the gate driver.
On the other hand, the liquid crystal display device comprises data drivers for producing the drive voltages OUT1, OUT2, . . . , OUT(n-1) and OUTn produced by converting the gradational voltage selected in accordance with the data signal of the display data into an appropriate voltage level and supplying them to a plurality of the second electrodes Y1 to Yn. The data drivers usually include upper positioned data drivers 600-1 to 600-N made of 4 to 5 ICs and lower positioned data drivers 610-1 to 610-N made of 4 to 5 ICs (N=4 to 5).
Further, the liquid crystal display device shown in FIG. 1 comprises a control signal generating circuit 300 for generating various control signals including a gate driver control signal Sxco and a data driver control signal Syco for controlling the operation of the gate driver 5, the upper positioned data drivers 600-1 to 600-N and the lower positioned data drivers 610-1 to 610-N in accordance with an external input signal Sin.
Generally speaking, when a DC drive voltage is applied continuously to the liquid crystal, the residual image and the contrast decrease, often deteriorating the quality of the image on display. In order to avoid this image quality deterioration, the liquid crystal in each pixel on the liquid crystal display panel 4 is supplied with such a drive voltage as not to generate any DC component on a temporal average or, for example, an AC drive voltage alternating between positive and negative polarities at regular time intervals. A voltage waveform of the drive voltage outputted from the upper positioned data drivers 600-1 to 600-N (hatched portion in FIG. 2) and a voltage waveform of the drive voltage outputted from the lower positioned data drivers 610-1 to 610-N (hatched portion in FIG. 2) are shown as a typical example of the AC drive voltage in FIG. 2. In this case, the drive voltage outputted from the upper positioned data drivers 600-1 to 600-N and the drive voltage outputted from the lower positioned data drivers 610-1 to 610-N, as viewed at the same time point (t), are related to each other in such a way that when one type of the drive voltage has a positive drive voltage waveform (plus drive voltage waveform), the other type of the drive voltage has a negative drive voltage waveform (minus drive voltage waveform). In this way, the polarities of the two types of drive voltages are opposite to each other. The amplitude and polarity of the drive voltages of the upper positioned data drivers 600-1 to 600-N and the lower positioned data drivers 610-1 to 610-N are controlled by the control signal outputted from the control signal generating circuit 300.
The liquid crystal display device shown in FIG. 1 further comprises a power source circuit 200 for generating a DC output power source voltage Vout required for activating the gate driver 5, the upper positioned data drivers 600-1 to 600-N and the lower positioned data drivers 610-1 to 610-N from an input power source voltage Vin of an external input power source. Further, a power source control circuit 310 is provided for generating a power source control signal Ss for setting an appropriate voltage level of the positive power source voltage (plus (+) power source voltage) +Vs and the negative power source voltage (minus (-) power source voltage) -Vs required for outputting the drive voltages OUT1 to OUTn from the upper positioned data drivers 601-1 to 600-N and the lower positioned data drivers 610-1 to 610-N.
The control signal generating circuit 300, the control circuit portion such as the power source control circuit 310 and the power source circuit portion such as the power source circuit 200 make up a drive circuit for the liquid crystal display device. This drive circuit is usually comprised of one or a plurality of ICs.
The magnitude of the absolute value of the voltage level of the positive power source voltage +Vs and the negative power source voltage -Vs, i.e. the voltage level of the power source voltage applied to the data drivers, as shown in FIG. 2, is normally set to be larger than the maximum amplitude of the drive voltage corresponding to the gradational voltage of the maximum voltage level. In other words, the data drivers are always supplied with a power source voltage higher than the maximum drive voltage in order to output a drive voltage of an arbitrary amplitude.
As described above, the conventional liquid crystal display device is always supplied with a power source voltage of a predetermined voltage level higher than the maximum drive voltage to control the data drivers in accordance with the drive voltage of an arbitrary amplitude. In using a notebook-sized personal computer having this liquid crystal display device built therein, assume that a panel (normal white) is used which is driven by a high voltage in the black display state and by a low voltage in the white display state. In white display state, the amplitude of the actually-outputted drive voltage normally assumes a value smaller than the maximum value of the drive voltage corresponding to the gradational voltage of the maximum voltage level.
In the prior art, however, a comparatively high power source voltage exceeding the maximum value of the drive voltage is normally applied to the data driver even in white display state when the amplitude of the drive voltage is lower than the maximum value thereof. Therefore, the current flows through a plurality of semiconductor devices in the data drivers. As described above, the data drivers include a plurality of ICs, and the total current flowing through the semiconductor devices in the ICs is not negligible. Consequently, unnecessary power consumption in the data driver occurs, thereby making it difficult to realize lower power consumption in the liquid crystal display device.